The thermal and pressure impingement effects of exhaust gases emanating from a rocket motor nozzle extend longitudinally well beyond the exit plane of the nozzle because of the concentrated supersonic flow pattern characteristic of the exhaust gas plume. Relatively small tactical missile rocket motors have nozzle exit velocities on the order of Mach 3.5 with recovery pressures of 300 pounds per square inch and recovery temperatures in excess of 5,000 degrees Fahrenheit. These high velocities, pressures and temperatures encountered in tactical or test firings of the rocket motor remain concentrated in the plume extending its length and result in destructive final pressure and thermal impingement effects in a relatively small area to the rear of the motor. In open environment static test firings, this necessitates strong and thermally protected structures positioned a considerable distance to the rear of the rocket nozzle, or a relatively vast open area behind the rocket. Test firings of the rocket motor into a duct or plenum require that similar features be incorporated in the plenum design to avoid destruction of the enclosure by the exhaust gas impingement. In tactical firings, the launching pad and components must also be protected. Provision of structures that can withstand the adverse temperature and pressure of the rocket motor exhaust increase the complexity and cost of the installations involved if firing safety is to be preserved.
It is desirable, therefore, to provide an apparatus to mitigate the thermal and pressure impingement effects. Applicant3 s invention transforms the initial supersonic exhaust gas velocity to one of subsonic (or much reduced supersonic) magnitude and provides for the dispersion of the exhaust plume to reduce these effects as well as shortening the effective length of a free exhaust gas plume.